(1) Field of the Invention
The present invention relates to a display device, and in particular, to a display device having a backlight system, such as a liquid crystal display device.
(2) Description of the Related Art
In display devices, such as liquid crystal display devices, the display panel is not a self-luminous display panel, unlike plasma display panels or organic electroluminescent panels, and therefore, the rear surface of the display panel is provided with a backlight system that functions as a surface light source.
In backlight systems for mid and small-sized liquid crystal display devices, a light guide plate or an optical sheet made of polycarbonate is sandwiched between the middle and lower frames in the backlight. In the design of this part with clearance, the member has a great coefficient of thermal expansion in the X direction (left and right directions of the display screen) and in the Y direction (upward and downward directions of the display screen), and therefore, a structure for suppressing unsteadiness is used. In contrast to this, the amount of expansion of the member is small in the Z direction (front and rear directions of the display screen), and therefore, no structure for suppressing unsteadiness has any great effects, and thus, none have been taken into consideration according to the prior art.
The clearance in the Z direction between the inner surface of the middle frame and the upper surface of the optical sheet is approximately 0.2 mm to 0.3 mm when the lower frame, a reflective plate, a light guide plate and an optical sheet are layered on top of each other in this order. When a certain vibration is applied to a conventional display device, the members within the backlight move in the Z direction, causing friction between the optical sheet and the edge around the opening of the middle frame, and thus, the optical sheet is scratched. In addition, the vibration causes such a problem that noise is created by the unsteadiness.
JP 2003-168311A discloses a technology for arranging and securing a light guide plate made of an acryl-based resin or the like within a frame made of a polyprene resin or the like. Concretely, it is disclosed that a recess is created inside the frame and a protrusion is formed in a predetermined location around the light guide plate, and thus, a rubber cap is placed in the gap between the recess and the protrusion in such a state that the two are engaged with each other.
In accordance with the securing method in JP 2003-168311A, however, the effects of suppressing unsteadiness in the Z direction, which is the front and rear directions of the display screen, is insufficient, though the unsteadiness of the display screen can be suppressed in the X direction and the Y direction.
Though a structure is also possible where the middle frame directly holds the main surface of the light guide plate, this still causes problems. For example, the following problems arise.
(1) A process for indenting the middle frame so that it reaches the level of the light guide plate is necessary.
(2) In place of the structure of the above (1), the light guide plate is provided with a protrusion that reaches inside the middle frame (a process for this is difficult).
(3) The structures of the above (1) and (2) cause scratches or allows foreign substances on the light guide plate due to the friction at the time of vibration.
(4) The crossing of parts makes the gap slightly larger, which causes unsteadiness.